Least favorite childish trait exhibited by a grown-up over the age of 40:

Attention-seeking behavior,

and/or being a showoff.*

How about you?

(*Like the guy in choir who has to stand and sing in highly enunciated German to show us that he knows Beethoven's 9th Symphony, like duh, we've all heard it a million times, while all the rest of us are sitting and singing "doo doo doo" to learn our parts.)